Zinc dithiophosphates (ZnDTP) have long been used as antiwear additives and antioxidants in engine oils, automatic transmission fluids, hydraulic fluids and the like. Conventional engine oil technology relies heavily on ZnDTP to provide extremely low cam and lifter wear and favorable oxidation protection under severe conditions. ZnDTP operates under mixed-film lubrication conditions by reacting with rubbing metal surfaces to form protective lubricating films. The mixed-film lubrication regime is a mixture of full-film (hydrodynamic) lubrication wherein the lubricating film is sufficiently thick to prevent metal-to-metal contact and boundary lubrication wherein the lubricating film thickness is significantly reduced and more direct metal-to-metal contact occurs.
However, a problem has arisen with respect to the use of ZnDTP, because phosphorus and sulfur derivatives poison catalyst components of catalytic converters. This is a major concern as effective catalytic converters are needed to reduce pollution and to meet governmental regulations designed to reduce toxic gases such as, for example, hydrocarbons, carbon monoxide and nitrogen oxides, in internal combustion engine exhaust emission. Therefore, it would be desirable to reduce the phosphorus and sulfur content in engine oils so as to maintain the activity and extend the life of the catalytic converter.
There is also governmental and automotive industry pressure towards reducing the phosphorus and sulfur content. As the environmental regulations governing tailpipe emissions have tightened, the allowable concentration of phosphorus in engine oils has been significantly reduced with further reductions in the phosphorus content of engine oils being likely in the next category, for example, GF-5 to perhaps 500 ppm.
However, simply decreasing the amount of ZnDTP presents problems because this necessarily lowers the antiwear properties and oxidation-corrosion inhibiting properties of the lubricating oil. Therefore, it is necessary to find a way to reduce phosphorus and sulfur content while still retaining the antiwear and oxidation-corrosion inhibiting properties of the higher phosphorus and sulfur content engine oils.
Accordingly, as demand for further decrease of the phosphorus content and a limit on the sulfur content of lubricating oils is very high, this reduction cannot be satisfied by the present measures in practice and still meet the severe antiwear and oxidation-corrosion inhibiting properties required of today's engine oils. Thus, it would be desirable to develop lubricating oils, and additives and additive packages therefor, having lower levels of phosphorus and sulfur but which still provide the needed wear and oxidation-corrosion protection now provided by lubricating oils having, for example, higher levels of ZnDTP, but which do not suffer from the disadvantages of the lubricating oils discussed above.